Most DC to DC switching voltage regulators such as the Buck converter and boost converter are capable of only regulating a voltage above or below a given input but not capable of both step up and step down regulation. A SEPIC (single ended primary inductor converter) is a DC-DC converter which allows the output voltage to be greater than, less than, or equal to the input voltage. The output voltage of the SEPIC is controlled by the duty cycle of the control transistor. The largest advantage of a SEPIC over the buck-boost converter is a non-inverted output (positive voltage). SEPICs are useful in applications where the battery voltage can be above and below the regulator output voltage. For example, a single Lithium ion battery typically has an output voltage ranging from 4.2 Volts to 3 Volts. If the accompanying device requires 3.3 Volts, then the SEPIC would be effective since the battery voltage can be both above and below the regulator output voltage. Other advantages of SEPICs are input/output isolation and true shutdown mode: when the switch is turned off output drops to 0 V.
As shown in FIG. 1 a prior art SEPIC converter 1 comprises a PWM control circuit 2, N-channel power MOSFET 3 with intrinsic drain-to-source diode 4, high-side inductor 5, capacitor 6, low-side inductor 7, rectifier diode 8 and output capacitor 9 powering load 10. Operation comprises repeatedly magnetizing inductor 5 whenever MOSFET 3 is in its ON and conducting state and transferring energy to output capacitor 9 and load 10 in alternating phases.
During operation, the node voltage Vx peaks at a voltage (VIN+VOUT). The BVDSS breakdown of MOSFET 3 and diode 4 must exceed this peak voltage with some reserve.
The converter as shown cannot survive an over-voltage condition because no means exists to stop the switching operation of MOSFET 3. Instead of limiting the maximum input voltage, converter 1 continues to operate at any input voltage until the drain voltage on MOSFET 3 exceeds safe limits and damages the device. In addition to this inability to survive high input voltages, PWM controller 2 contains low-voltage control circuitry which cannot operate when powered directly from a high voltage input.
The circuit as shown also suffers from the lack of a true load disconnect. Current sensing is also problematic since there is no convenient means to detect the input current in inductor 5.
What is needed is a SEPIC converter offering high-voltage operation up to some safe level below the rating of the power MOSFET, a means to inhibit switching operation under excessive input voltage conditions, the ability to disconnect the load from the input, and a means to detect the input current either to implement current mode control, to prevent over-current conditions, or ideally both.